1. Field of the Invention
This invention relates to an ultrasonic imaging apparatus such as a medical electronic-scanning ultrasonic diagnostic apparatus.
2. Description of the Related Art
For medical electronic-scanning ultrasonic diagnostic apparatus, various types of ultrasonic probe units are used depending on clinical and/or technical conditions. Typical examples of clinical conditions include the shape and location of a visual field region or a target region. Typical examples of technical conditions include the number of arrays of transducers and the natural frequency of each transducer. According to the clinical and/or technical conditions required by the operator, a diagnostic format, such as B-mode display (tomographic image display), M-mode display (motion image display), D-mode display (Doppler image display), BDF-mode display (superposed display of a tomographic image and a flow mapping image), MDF-mode display (superposed display of a motion image and a color mapping image), linear scanning, sector scanning, and convex scanning, is carried out.
The probe unit used in such an ultrasonic imaging apparatus as a medical electronic-scanning ultrasonic diagnostic apparatus contains a transducer section consisting of arrays of transducers. This transducer section is applied to the subject. When the transmission system is activated, the transducer section transmits an ultrasonic beam to the subject and then receives reflected waves generated within the subject. Based on the received waves, the transducer section produces and supplies a reception signal to the reception system. The transmission system and reception system are based on scanning control such as electrical scanning. The reception signal entering the reception system is processed at the signal processing system to produce display information such as a tomographic image.
In the above medical electronic-scanning ultrasonic diagnostic apparatus, electronic scanning control and beam deflection control are achieved by performing the transmission driving and reception driving of the arrays of transducers in the transducer section, each transducer being assigned a different amount of delay. Such control is known as phase control. Since electronic ultrasonic scanning provides phase control of arrays of transducers, scanning control accuracy and display information precision are dependent largely on the phase characteristics of the transducer section. The transducer section composed of transducers of identical characteristics arranged with high accuracy would provide excellent phase characteristic. In the manufacture of actual transducers, however, it is natural that there is a limit to obtaining a plurality of transducers of identical characteristics and arranging them with high accuracy. As a consequence, it has been impossible to obtain satisfactory ultrasonic images.
The operating conditions such as an excitation frequency vary with the type of a probe unit used. In addition, as mentioned above, the transducers constituting the transducer section each have different phase characteristics. Therefore, for example, when a tomographic image is displayed with the main body connected to a linear scanning probe unit and then to another different in the specifications from the first, no satisfactory ultrasonic image is obtainable even if a specified excitation frequency is set for each case, because the probe units differ from each other in terms of phase characteristics.